1. Field of the Invention
The present invention relates to an apparatus and a method for detecting particles on an object. More particularly, the present invention relates to an apparatus and a method for detecting particles on an object such as a wafer by irradiating a light to the particles.
2. Description of the Related Art
Generally, as the dimensions of a semiconductor device has been reduced, and the degree of integration of the semiconductor device has been augmented, it has required that the detection level for monitoring particles be improved. Since a cell size of the semiconductor device has been reduced, and also the size of the particles having an adverse influence on the electrical characteristics of the semiconductor are small, an apparatus capable of detecting a particle having a size of below about 0.1 μm has been developed and is used currently. The apparatus generally detects these particles using laser scattering techniques.
There is an issue concerning an ability of an apparatus in particle monitoring to distinguish the particle on a wafer from a critical oriented particle (COP). The COP serves as an initial defect caused by abnormality of crystalline structure of the wafer that is formed on the wafer in a V shape.
It is rare that the COP occurs on an initial-fabricated wafer. Accordingly, distinguishing the actual particle on the wafer from the COP in the initial-fabricated wafer is carried out without a problem. The wafers are, however, recycled due to high cost of using the wafer only once. When the used wafer is recycled, the COP level on the used wafer increases. The COP is readily distinguished from the actual particle on a bare wafer. However, when an oxide layer or a nitride layer is formed on the bare wafer, the COP is not readily distinguished from an actual particle on the bare wafer.
A conventional method for inspecting on a wafer is disclosed in Korean Laid Open Patent Publication No. 2001-0086099. In the conventional method, a light is reflected or scattered from a surface of a wafer. Light-receiving units receive the reflective or scattered light. A shape and a category of defects on the wafer are inspected according to ratio of light-receiving intensity in the light-receiving units.
FIG. 1 is a cross sectional view illustrating a conventional apparatus for detecting particles on a wafer.
With reference to FIG. 1, a conventional apparatus has two emitters and two detectors. The emitters include a first emitting member 10 for irradiating a first light having an incident angle of about 70° to a wafer W and a second emitting member 12 for irradiating a second light having an incident angle of about 9° to the wafer W. The detectors include a first detecting member 14 for detecting a first scattered light having a wavelength in a wide band reflected from the wafer W and a second detecting member 16 for detecting a second scattered light having a wavelength in a narrow band reflected from the wafer W. When the wafer W is a bare wafer. The COP has a shape different from that of the actual particle on the bare wafer. Thus, a signal detected in the detectors 14 and 16, which corresponds to the lights reflected from the COP, may be distinguished from a signal detected in the detectors 14 and 16, which corresponds to the lights reflected from the actual particle on the wafer W. Namely, the actual particle on the wafer and the COP may be distinguished using the difference between the respective signals.
FIG. 2A is a graph illustrating a ratio of a light having a wavelength in a narrow band relative to a light having a wavelength in a wide band of the light. With reference to FIG. 2A, the COP and the actual particle on the wafer are distinctly distinguished on the basis of a slope line having a ratio of about 1.5. The ratio corresponds to a ratio of the light having a wavelength in a narrow band detected by the second detecting member 16 relative to the light having a wavelength in a wide band detected by the first detecting member 14. Accordingly, when the bare wafer is monitored through irradiation of the lights from the first and second emitting members 10 and 12, the COP and the actual particle may be accordingly distinguished.
It is, however, that after an oxide layer or a nitride layer is formed on the bare wafer, the COP may be difficult to distinguish from the actual particle because the COP has a shape similar to that of the actual particle. Thus, the difference between the signals detected by the detectors 12 and 14 is hard to distinguish. As a result, the COP may not be distinguished from the actual particle according to the method using the difference between the signals.
FIG. 2B is a graph illustrating a ratio of a light having a wavelength in a narrow band relative to a light having wavelength in a wide band of the light when the oxide layer is formed on the bare wafer. FIG. 2B shows the above-mentioned result. In particular, when the wafer is a recycled wafer, the number of the COPs increases proportional to the number of recycling of the wafer. Accordingly, when the particles on the recycled wafer are monitored, the COP may not be distinctly distinguished from the actual particles so that the particles may not be detected.
A method and an apparatus for detecting particles are disclosed in Japan Laid Open Patent Publication No. 1999-284038. In the method and the apparatus, a light is irradiated to a surface of a silicon wafer. Detectors receive a light scattered from the surface of the wafer in front, rear and upward directions. Although the scattered light is feeble, the COP may be distinguished from the actual particle so that the actual particle may be detected.
FIG. 3 is a cross sectional view illustrating an another conventional apparatus for detecting particles on a wafer.
Referring to FIG. 3, the apparatus includes three detectors having a front channel 32, a center channel 34 and a rear channel 36. The light emitted from a light source 30 is reflected and scattered from a wafer W disposed on a stage 38. The detectors 32, 34 and 36 detect the scattered lights so that the actual particle may be distinguished from the COP using a difference between signals of detected lights.
A significant portion of the lights scattered from the actual particle disposed on the wafer W are mainly oriented to the front and rear channels 32 and 36. The remaining lights are oriented to the center channel 34. On the other hand, the lights scattered from the COP are uniformly oriented to the front, center and rear channels 32, 34 and 36.
To classify the COP on the bare wafer, the following algorithms may be used.CM>1.14′RMCM>1.36′FM
In the above algorithms, CM represents a magnitude of a signal outputted from the center channel 34. RM represents a magnitude of a signal outputted from the rear channel 36. FM represents a magnitude of a signal outputted from the front channel 32.
When CM is above 1.4 times RM and is simultaneously above 1.36 times FM, this defect may be classified as the COP. On the contrary, when CM is below 1.4 timed RM and is simultaneously below 1.36 times FM, this defect may be classified as the actual particle.
The above results are illustrated in FIGS. 4A and 4B. FIG. 4A is a graph illustrating a ratio between CM and RM. FIG. 4B is a graph illustrating a ratio between CM and FM.
However, as described above, after the oxide layer is formed on the bare wafer, the relation between the COP and the actual particle does not entirely exist. These results are illustrated in FIGS. 4C and 4D. FIG. 4C is a graph illustrating a ratio between CM and BM. FIG. 4D is a graph illustrating a ratio between CM and FM.
Although the conventional apparatus may distinguish the COP from the actual particle on the bare wafer, the COP may not be distinguished from the actual particle on the wafer having the oxide layer or the nitride layer when employing the above-described conventional apparatus.